Note: Descriptions are shown in the official language in which they were submitted.
CA 02576071 2007-02-05
THERMOLYSIS OF ORGANIC WASTE IN A BALL FURNACE
Technical domain
The invention relates to the treatment of organic
waste consisting of industrial waste, agricultural waste
or household waste. It relates to their transformation by
thermolysis, and particularly by thermolysis taking place
in an installation in which a fixed or rotating furnace
is installed.
Waste elimination operations are performed in a
context of reuse and preservation of the environment.
Thermolysis is a waste elimination process that offers an
alternative to incineration, over which it has many
advantages (no emission of dioxins, no production of ash
contaminated by organic compounds, excellent operating
flexibility). A good presentation of the question is
given in the "Report on new technologies for reuse of
household waste and non-hazardous industrial waste",
Parliamentary Office for the Evaluation of Scientific and
Technological Choices, France, National Assembly No. 1693
/ Senate, No. 415, G. Miquel and S. Poignant (Part II:
treatment processes, III: Thermolysis), and in G.
Poulleau, 'Household waste', Air Eau Conseil edition,
2001.
Prior art
Thermolysis consists of a chemical decomposition by
heating organic matter in any form whatsoever (liquid,
paste or solid) in the absence of air. It is done
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continuously or discontinuously by increasing the
temperature of the organic matter to 400 C to 700 C, or
possibly even 1100 C when the objective is to treat risk
waste in a fixed or rotating containment in the absence
of air. (The term 'risk waste' should be understood to
have the meaning defined by the lawmaker, particularly
including bovine parts that could contain prions, BSE
propagation agents). The production of charcoal, that is
usually referred to as pyrolysis, is a thermolysis.
Pyrolysis has been used for many years in the past for
the reuse of household waste (DE 29040324, Berghoff).
Regardless of the production process, thermolysis
transforms organic substances into products that can be
reused in different ways:
- gases burned on the production site, for example
as a heat source for the thermolysis itself;
- condensates, for which the oil fraction can be
exported as a fuel;
- solid residue firstly including a coke, exportable
as a fuel after reprocessing, and a mineral fraction that
can be reused or eliminated in accordance with the
legislation in force, depending on the nature and
characteristics of the organic matter being treated.
The mass to be thermolysed is heated by various
means, including direct action of a radiant flame.inside
the containment, circulation of fumes or combustion gases
through the mass of waste to be thermolysed, contact with
internal tubes, external heating of the containment.
There are many descriptions of thermolysis installations,
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for example in patents FR 2654112 (CGS), FR 2725643
(Traidec).
Direct heating of the matter with a hot combustion
gas is a technique that has serious disadvantages.
Combustion gases usually produced with the thermolysis
gas contain large quantities of oxygen. Producing a flame
in a containment in which it is required to thermolyse
the waste requires that the quantity of air in the flame
should be increased so as to maintain this flame and its
temperature that inevitably drops in a reducing medium.
100% to 200o excess air is frequently observed under
these conditions, and consequently the excess air and
particularly the oxygen in this air will combine with
molecules containing chlorine (for example) to produce
dioxins and also all other sorts of combinations that
denature the thermolysis products. Other disadvantages
should be mentioned, particularly lowering of the net
calorific value (NCV) of the gas output from the
thermolysis containment, and the obligation to treat non-
recycled fumes particularly to eliminate unburned
products following burning of a gas containing fumes.
This is true for all waste destruction processes that
take place in the presence of air, such as the De Muynck
process described in US patent 5 762 010 that is related
to a fluidised bed combustion process in which the waste
entrained by ceramic balls is burned completely in the
same containment after having been partially pyrolysed
under the effect of heat released by this combustion.
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External heating of the containment requires large
heat exchange surface areas, and a relatively long
residence time of organic matter in the containment; the
thermal efficiency is affected by the loss of calories in
the fumes; overheating of the walls causes them to
collect dirt more or less quickly on the inside, and
catalytic or mechanical cleaning systems are more or less
efficient at cleaning them during compulsory shutdowns.
Heating tubes internal to the furnace are very
sensitive to degradation by badly shredded scrap,
inevitably added sooner or later with the waste to be
treated. No waste thermolysis processes are capable of
reaching temperatures greater than 700 C, unless a flame
is applied in direct contact, like in an incineration
process. Consequently, it is impossible to offer
satisfactory solutions for some waste such as hazardous
waste from bovine slaughterhouses or healthcare waste.
Presentation of the invention
This invention overcomes these disadvantages with an
organic waste thermolysis process that consists of adding
the heat necessary for heat treatment of this waste using
previously superheated steel balls.
For the purposes of this invention, the term
'organic waste' applies to different solid, semi-paste or
paste bodies containing a certain proportion of organic
matter. The following is a non-limitative list of such
matter:
rottable fraction of household waste,
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sludge from industrial effluent and urban sewage
treatment plants,
farm waste, composting refuse,
organic m~atter from the agro-food industry (grease,
5 slaughterhouse waste including risk waste, animal flour,
etc. ) ,
non-reusable industrial organic matter,
used non-retreadable shredded tires,
healthcare waste,
in general, all waste containing organic matter which, if
the legislation is respected, can no longer be buried as
such or incinerated. Note that the existing legislation
does not allow burial of hazardous industrial waste
unless its total organic carbon content is less than 3
grams per kg; this demonstrates the advantage of
processes like the process according to the invention
that is capable of eliminating all organic carbon in
waste with low organic content such as sand contaminated
by hydrocarbons or phenols.
This definition is also applicable to organic
liquids that can be distributed on the balls as a
coating, or possibly mixed beforehand with absorbent
organic supports, for example vegetable waste or sawdust.
Also according to the meaning of this invention,
'thermolysis' means heat treatment in the absence of air
leading to physical and chemical transformation of the
thermolysed matter with the release of condensable or
incondensable volatile products and the formation of a
solid carbonaceous residue (coke). This is a genuine
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thermolysis, which can only take place in complete
absence of air. This is how the process is different from
a combustion or partial thermolysis waste treatment.
Using these definitions, the invention consists of a
process for heat treatment of organic waste in an oxygen-
free atmosphere, in which the waste is heated in a fixed
or rotating furnace, characterised in that the means of
heating the said waste consists of previously superheated
steel balls and that move forwards in the furnace at the
same time as the said waste with which they are
intimately mixed. It also consists of an installation for
the heat treatment of organic waste comprising at least
one fixed or rotating furnace in which waste moves
forwards during its treatment, means of supplying the
furnace with waste, means or recuperating the treated
waste, means of recuperating volatile products derived
from this treatment and means of heating the waste mass,
characterised in that the means of heating the waste mass
consists of a mass of previously superheated steel balls
that move inside the furnace with the waste to be
treated, and devices for supplying the furnace with
superheated balls, for recuperating them at the exit from
the treatment furnace so that these balls can be
recirculated, and a furnace for heating the balls.
Figures and references
The general structure of a thermolysis group
according to the invention is shown diagrammatically in
Figure 1, wherein:
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(1) Thermolysis furnace;
(2) Entry duct for balls heated to high temperature;
(3) Inlet duct for waste to be thermolysed;
(4) Exit drum or lock for balls that have released
their thermal energy;
(5) worm screw;
(6) Vibrating grating to separate the balls from
thermolysis residue;
(7) Exit duct for the mix of balls from thermolysis
residue;
(8) Duct for recirculating steel balls;
(9) Steel balls;
(10) Steel ball heating furnace;
(11) Steel ball entry drum or lock;
(12) Entry drum or lock for material to be treated;
(13) Vacuum pump extraction fan for suction of
thermolysis gases and for keeping a slight negative
pressure in the furnace;
(14) Manifold (thermolysis gas for reuse as energy;
sludge for condensation and extraction of incondensables
to be burned);
(15) Thermolysis residue recuperation hopper;
(16) Thermolysis residue transfer screw;
(17) Evacuation lock or drum for thermolysis
residue;
(18) Thermolysis residue evacuation duct;
(19) Makeup heating to compensate for furnace losses
or adding energy during start up phases;
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(20) Insulation of the furnace, ducts, hopper, etc.
assembly,;
(21) Vacuum condensation assembly, recuperation of
thermolysis condensables;
(22) Device for lifting the heating mass (screw,
belt, etc.);
(M) are motors.
Figure 2 shows a specific thermolysis unit; the
following elements can be seen, that are complementary to
the elements shown in the general Figure 1:
(31) Thermolysis residue storage hopper;
(32) Thermolysis residue washing tank;
(33) Solid residue evacuation duct;
(34) Coke dripping belt;
(35) Coke drip storage and recovery tank;
(36) Storage tank for all liquid effluents,
condensates, coke drips, dirty water from the washing
basin and separation of thermolysis residue;
(37) To use of thermolysis gases.
Figure 3 shows a diagram of an installation that
could collect waste with variable humidity, that
comprises a drying furnace and a thermolysis unit in
series. It includes the elements indexed on the previous
figures around:
(S) the drying furnace;
(T) the thermolysis furnace;
(24) a buffer hopper for storage of dried waste;
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(38) an elevator between the hopper (24) and the
thermolysis furnace inlet duct (3).
Production of the invention
Characteristically, the heating mass in the process
is composed of a large quantity of steel balls, usually
balls with a diameter of 20 to 50 millimetres. Larger
diameter balls can be used to treat special contents, for
example with a diameter of 60 millimetres to thermolyse
ground tyres or long-fibre waste. The choice of steel
balls provides a solution to some technical constraints,
particularly fast transfer of heat at high temperatures,
optimisation of exchange surface areas within the small
volume of the thermolysis furnace, mechanical
disintegration of the organic matter as soon as it enters
the furnace, and coke at the end of the path. The content
of balls in terms of mass and diameter is determined as a
function of the powers to be used, and the free volume
within the heating mass; there are other criteria such as
their manipulation or handling during recirculation and
particularly during transit in drums, and the concern to
avoid deformations in the thermolysis containment when
they drop in at the entry to the device. Example 1
contains information useful for estimating their content.
Their apparent density is high compared with the matter
to be treated and is of the order of 4000 kg/m3 to
4500 kg/m3. The developed surface area of the heating
mass compared with its volume is very high, such that
heat will be uniformly distributed in the waste mass when
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it is mixed with the matter to be treated. This special
feature is particularly appreciable when the objective is
heat treatment of hazardous waste: temperatures as high
as 1100 C are essential for total destruction of protein
5 material and therefore prions only in installations in
which the thermolysis temperature is not uniform.
The furnace in which heat treatment of the waste
takes place is a horizontal or slightly inclined furnace.
When the power to be applied is relatively modest and the
10 mass to be treated is not much more than
500 kilograms/hour, the furnace in which the heat
treatment of the waste takes place is preferably a fixed
furnace in which the balls + waste mass moves forward
under the act-i_on of a worm screw (5) fitted with mixing
devices (for example profiled bars). This is the
embodiment that has been used as a descriptive example
for the figures, although this in no way restricts the
scope of the invention. For high capacities, the furnace
will more often be a traditional furnace equipped with a
balls and waste pre-mixing device at the ball and waste
entry.
Makeup heating (19) is provided, if only for
preheating of the furnace when the installation is
started up; it occasionally fulfils various functions;
maintaining the exit temperature of the steel balls,
makeup when changing conditions (flow of materials, rise
in the thermolysis temperature, drying, etc.).
All entry and exit drums and locks for materials are
air tight by construction. In practice, they are provided
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with pressure balancing units to neutralise the inside
volume of the drum and the waste supply or the exit of
solid thermolysis products is made through a cascade of
hoppers with automatic filling. All rotating parts,
bearings in the rotating furnace, the shaft of the
Archimedes screw and the ball and solid lifting and
transfer screws are also made impermeable to air, for
example by installing the motors and bearings in sealed
cages.
The process operates as described below (refer to
Figures 1 and 2) . Waste penetrates into the thermolysis
furnace (1) through the inlet duct (3) and the drum (12)
and they meet the steel balls inlet at the top through
the duct (2) and the drum (11) from a heating furnace
(10) in which their temperature was increased to the
order of 600 C to 1100 C.
Thermolysis takes place during mixing of the waste
and balls as the materials move forwards in the furnace
(7). Materials leaving the furnace are now composed of
cooled balls and thermolysis residue. Their temperature
is then between 500 C and 850 C. The thermolysis residue
is extracted through the grating (6), collected in a
recuperation hopper (15) and taken outside through an
extraction system (16) and a lock (17) and duct (18).
Balls are recovered through the drum (4), returned
through the elevator (22) and sent through the duct (8)
to the furnace (10) where they resume their cycle.
Thermolysis gases are captured by a manifold (14),
separated from their condensable components (21) and
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extracted in (13) to be burned or to supply gas turbines
on site.
The installation (Figure 2) is completed with
elements that were listed with reference to Figure 1 and
with which those skilled in the art are familiar. (32) is
the equipment for compulsory treatment of the thermolysis
residue, immersion to separate a floating combustible
material (coke) by dripping (34), solid residue by
settlement (33) including metals that can be separated
for example by metallic sorting or by Eddy currents, and
immersion and dripping water (35) to be sent to a storage
tank (36) for a depollution treatment. The settled sludge
(33) will be treated or conditioned before evacuation to
a specialized burial centre or for reuse if the final
product is acceptable.
Thermolysis gases can be used on site (37).
The waste increases in temperature very suddenly on
contact with the steel balls, which facilitates
production of gas instead of coke. Gases released at high
temperature then remain in contact with the heating mass
for long enough to crack greases and other heavy
molecules generated in some types of waste: a thermolysis
gas with an optimised calorific value is produced,
therefore the collection of dirt in the installation is
reduced.
Steel balls are heated in a gas, electrical
radiation or induction furnace (10) . If a gas furnace is
used, it is advantageous to use thermolysis gas drawn off
from production at a percentage varying between 10% and
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15%, so that 85% to 900 of gases will be available for
external reuse as energy. The simplest process is a bare
flame furnace, in which the atmosphere is isolated from
the remaining atmosphere in the installation by sealed
drums, as described above, to prevent the ingress of
excess air from the flame into the thermolysis furnace.
Heating of balls by induction is one particularly elegant
variant, and is possible due to the metallic nature of
the balls.
Since the process is simple and safe in use and
operation in continuous or semi-continuous mode, very
modest sized installations can be set up in the locations
in which waste is produced; they enable the waste
producer to destroy his own waste and recover excess
energy for his own purposes in the form of hot water,
steam or electricity.
For some applications in which power and flow
variations can occur, a storage hopper can be used for
the heated balls on the inlet side of the drum (11).
Furthermore, this hopper can be used as a reactor for
cracking thermolysis gases, for example if the waste is
liquefied grease.
As soon as the water content of the waste to be
thermolysed is high, the vaporisation of water becomes a
limiting factor for the thermolysis procedure and it is
better to dehydrate this waste in advance. The invention
is particularly conducive to such dehydration even within
the installation, at least if the initial dryness
(content of dry matter) is greater than 35%. (Below 35%,
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thermolysis would require an external input of calories
and it is more reasonable to apply an external mechanical
treatment requiring much lower energy to this matter
containing larger quantities of water).
This is shown diagrammatically in Figure 3. The
installation then comprises two furnaces of the same type
in series, the thermolysis furnace (T) and the drying
furnace (S). The same heating mass of steel balls
transits in these two furnaces and firstly thermolyses
the dried waste and then dehydrates the wet waste. The
wet waste enters into the drying furnace (S) and then
passes directly into the thermolysis furnace (T). The hot
heating mass firstly enters into the furnace (T) where it
thermolyses its content; its temperature is still
sufficiently high at the exit from the thermolysis
furnace to apply partial preliminary drying of the waste
in the furnace (S). An elevator (38) picks up the dried
waste at the hopper (24) and takes it to the thermolysis
furnace waste inlet lock (12). A gas and drying sludge
collection device is provided on the furnace (S) that
directs it towards the thermolysis gas burner.
Obviously, the installation according to the
invention could be used as a simple material drying
installation. Such drying, although not very
conventional, has several advantages, namely that since
it is done in the absence of air, it does not form any
dangerous oxidation products; and since it makes direct
contact between the matter to be dried and the heating
mass, the necessary energy is transmitted to the core of
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the matter causing a fast and uniform increase in its
temperature and prevents its agglomeration. Differences
between a thermolysis group and a drying group according
to the invention are minimal and take account of inlet
5 and outlet temperatures of the heating mass,
- in thermolysis, 600 C to 11000C at the inlet,
500 C to 850 C at the outlet for gases and residue,
- in drying, 500 C to 600 C at the inlet, 120 C to
140 C at the outlet of the dry waste,
10 for example, such that the condensation assembly
(21) is used for condensation of vapours to extract
incondensables to be burned in a boiler, or the
extraction fan (13) is used for drawing in vapours from
drying.
15 The thermal shock at the inlet is insufficient to
prevent any risk of prions being entrained in the vapours
for high risk waste.
Other uses could be envisaged for the installation
with a steel ball furnace (sterilisation, baking, etc.),
that remain within the scope of this invention.
The following non-limitative examples illustrate the
invention.
Example 1
A continuous thermolysis installation treating 800
tonnes annually (namely about 100 kg per hour) of waste
after being previously dried to reduce the water content
to 5 o, and titrating 700 organic matter (average
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composition), is arranged around a 0.7 m diameter tubular
reactor with a total length of 7.2 m.
Energy needs for the thermolysis determined by
preliminary tests are 50 kWh for 100 kg of waste
(excluding thermal losses). The average thermolysis
temperature is fixed at 600 C.
The heating mass is composed of 20 mm diameter steel
balls, the mass of which is estimated as follows.
With an average specific heat of steel equal to
0.174 W/kg/ C, the heating mass transferring its heat
from 700 C to 500 C is 50 000 / (0.174 x 200) = 1437 kg,
in other words 44 000 20 mm diameter balls (32.65 g per
ball).
The installation produces gas at a rate of about
70 kg per hour capable of generating about 600 kWh, and
kg of solid residue.
Example 2
The same installation can be used to process
20 butcher's waste. It guarantees thermolysis throughout the
mass at a temperature of 700 C, and possibly 900 C for
hazardous waste, the temperature at which all proteins,
including any prions are destroyed.
The possibility of direct treatment of butcher's
25 waste eliminates a step to transform the material into
animal flour.
The waste treatment process according to the
invention is a particular application of a more general
heat treatment principle, namely a process for submitting
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a divided solid or paste material to a heat treatment
(heating or cooling) in order to modify its physical
state or its chemical composition, characterised in that
the material to be treated enters a containment with
reverse current with a mass of steel balls previously
heated to a temperature such that the treated material
and the mass of balls at the exit from the containment
are at the chosen temperature for the heat treatment.
